Galaxies M81, M82 and the Ultra-Faint Integrated Flux Nebula

Galaxies Messier 81 and Messier 82 are two bright galaxies in the constellation
Ursa Major and are surrounded by the extremely faint Integrated Flux Nebula. M81 is on the
right and M82 is to the left. Objects are identified in the figure below.

M81 is a spiral galaxy some 12 million light years away. M82 is also about
12 million light years and is undergoing massive starbursts in its core.
It is the closest starburst galaxy to the Earth. Both M81 and M82 are nice sights
visually is modest-sized amateur telescopes (6-inches or more in aperture).

The Integrated Flux Nebula, or IFN, is composed of dust particles,
hydrogen, carbon monoxide and other gases, and is located at high
galactic latitudes above the galactic plane. The light is reflected
starlight from many stars in the Milky Way, thus the term integrated.
The nebula is very faint and is a challenge to image (photometric data
below). There is extended red emission from the gases in the nebula,
plus blue reflected light from the dust, which can give varied blue,
red and purple colors.

This is a natural color image. The high dynamic range of
astrophotos must be stretched to bring out the range of details the camera
recorded. But the typical image stretch process loses color for brighter
subjects (e.g. stars and the brighter parts of deep sky objects become
whiter as they are made brighter). This image uses a new algorithm,
rnc-color-stretch that does not lose color during the stretch.
How do we know the colors are reasonable? The star colors can be checked
against stellar photometry. Red stars have B-V > 2,
orange stars have B-V of 1 to about 2. The blue-white
stars have B-V in the range of 0 to -0.5. The colors closely follow
the color sequence in Table 1 at
Color of Stars. Solar-type stars have a B-V of 0.63 and appear close to white
(daylight white balance).

Technical.
Canon 7D Mark II 20-megapixel digital camera
and
300 mm f/2.8 L IS II
at f/2.8.
Forty seven 60-second exposures at ISO 1600 were added (47 minutes total exposure).
The Integrated Flux Nebula nebula is extremely faint (see below), so this
image demonstrates that the standard 7D Mark II is an amazing low light
camera for it to record such faint detail in this exposure. Full resolution
image is at 2.8 arc-seconds per pixel, and the image here is 1/3 that
(8.4 arc-seconds per pixel).
No dark frame subtraction, no flat fields. Tracking with an astrotrac and no guiding.

Photometry using the camera data shows that the sky brightness (camera RGB) was:red = 21.3, green = 21.8, blue = 22.1 magnitudes / sq. arc-second.
This is about the darkest sky I have imaged in during this solar cycle.
I measured the IFN above M81 at:red = 24.4, green = 25.8, blue = 25.4 magnitudes / sq. arc-second.
The sensor was at 11 C with dark current of 0.015 electron/second (0.9 electron/exposure),
and read noise was 2.4 electrons.
At these levels, the green channel was receiving about 38 photons from the sky,
and only 0.9 photon/pixel/exposure on average from the "bright" IFN patch above M81 with
fainter patches less than 1/3 that level. Thus, each pixel had about 6.7 electrons of
noise from the sky + dark current + read noise per exposure. So the bright parts of the IFN
had a signal-to-noise ratio of 0.13 and fainter parts only 0.04 per exposure.
This demonstrates the 7D2 with no dark frames, bias or custom flat fields can detect
signals less than S/N = 0.1 per exposure and averaging less than 1 photon
per pixel per exposure. The faintest parts of the image are at about
magnitude 27 per square arc-second.

The Exposure Factors, CEF, CEFA
are measures of the relative amounts of light received from a subject.
It can be used to fairly compare wildly different lens/telescope apertures and exposure times.
For this image:

Objects in the field labeled. The arrows with no labels shows faint galaxies for which
I have not been able to find identifications.

Modern DSLRs like the 7D Mark II include
On-Sensor Dark Current Suppression Technology
and low fixed pattern noise at ISOs around 1600 and higher, making no
need for dark frame subtraction. Modern raw converters correct for light
fall-off (flat field correction on linear sensor data) and also correct
for hot/dead/stuck pixels. This makes processing low light images
easy: simply align and average. See my series on Astrophotography Image Processing for more details.